LTC3499_15 Datasheet, PDF (10/16 Pages) Linear Technology – 750mA Synchronous Step-Up DC/DC Converters with Reverse-Battery Protection

LTC3499/LTC3499B
APPLICATIONS INFORMATION
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3499/LTC3499B
demand careful attention to board layout. Advertised per-
formance will not be achieved with careless layout. Figure  2
shows the recommended component placement. A large
copper area will help to lower the chip temperature. Traces
carrying high current (SW, V_OUT, GND) are kept short.
The lead length to the battery should be kept as short as
possible. The V_IN and V_OUT ceramic capacitors should be
placed as close to the IC pins as possible.
EXPOSED PAD FOR DD8
SHDN ¹
8 ^VC
C_C2
C_C1
RZ
V_{IN 2}
7 ^FB
R2
9
+
C_IN L
R1
V_BATT
SW 3
6 ^VOUT
GND 4
SS
5
C_OUT
C_SS
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Figure 2: Recommended Component Placement
COMPONENT SELECTION
Inductor Selection
The LTC3499/LTC3499B allow the use of small surface
mount inductors and chip inductors due to the fast 1.2MHz
switching frequency. A minimum inductance value of
2.2µH is required. Larger values of inductance will allow
greater output current capability by reducing the induc-
tor ripple current. Increasing the inductance above 10µH
will increase total solution area while providing minimal
improvement in output current capability.
The inductor current ripple is typically set to 20% to 40%
of the maximum inductor current. For high efficiency,
choose an inductor with high frequency core material,
such as ferrite, to reduce core losses. The inductor should
have low ESR (equivalent series resistance) to reduce the
I²R power losses, and must be able to handle the peak
inductor current without saturating. To minimize radiated
noise, use a toroidal or shielded inductor. See Table 1 for
some suggested inductor suppliers.
Table 1. Inductor Vendor Information
PART NUMBER
SUPPLIER
MSS5131 and
MOS6020 Series
Coilcraft
SLF7028 and
TDK
SLF7045 Series
LQH55D Series
Murata
CDRH4D28 Series
Sumida
D53LC and
Toko
D62CB Series
DT0703 Series
CoEV
MJPF2520 Series
FDK
WEB SITE
www.coilcraft.com
www.component.tdk.com
www.murata.com
www.sumida.com
www.tokoam.com
www.coev.net
www.fdk.com
Output Capacitor Selection
The output voltage ripple has three components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The maximum
ripple voltage due to charge is given by:
( ) ^{VRBULK = IP •}
V_IN
C_OUT • V_OUT • f
where I_P = peak inductor current and f = switching
frequency.
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is simply given by:
V_RCESR = I_P • C_ESR
where C_ESR = capacitor equivalent series resistance.
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